Vehicle steering is generally controlled by a driver hand wheel that directs the angle of road wheels. Movements of the driver hand wheel are typically transmitted to the road wheels by mechanical linkages and/or electronic components. The angle of the road wheels is commonly referred to as “road wheel angle”.
Active steering is a term referring to the use of electronic components to actively control or assist the steering of a vehicle so as to enhance steering performance beyond that possible by only direct mechanical linkages. There are many possible ways to enhance steering performance. For example, steering can be adapted to the weather conditions and/or to the behavior and/or habits of the driver. As another example, steering can be adapted to provide improved directional control during braking.
Open loop control refers to the operation of a road wheel angle controller, without feedback, that is independent of any supervisory control system. For example, in an active steering system, variable gear ratio steering is a method for determining a target road wheel angle implied by a driver's hand wheel input. Such variable gear ratio active steering functionality typically uses variables such as a steering wheel angle, a steering wheel angle rate, and a speed of the vehicle to determine the target road wheel angle implied by a driver's hand wheel input. The active steering system may use variable gear ratio active steering functionality to ascertain the target road wheel angle for open loop control. The active steering system may then use an actuator to rotate the front road wheels for a given road wheel angle.
Closed loop control or electronic closed loop control refers to the operation of a road wheel angle controller with feedback from a supervisory control system. For closed loop control, an angle offset from a supervisory control system is added to the variable gear ratio steering angle to determine a target road wheel angle that should be implemented by the steering mechanism. For example, the active steering system may include an electronic stability control functionality that determines such an angle offset when the vehicle is in a skid, or when conditions otherwise warrant electronic stability control. The active steering system may then similarly use an actuator to rotate the front road wheels for a given road wheel angle adjustment.
Often, an active steering system will use a primary active steering functionality (such as an open loop, variable gear ratio functionality as described above) along with a supplemental active steering functionality (such as a closed loop, electronic stability control functionality as described above). The primary and supplemental active steering functionalities generally work well together. However, in certain situations it may be necessary to discontinue use of the supplemental active steering functionality. When this occurs, typically the actuator is locked, in order to bypass the supplemental active steering functionality. However, this also has the effect of bypassing the primary active steering functionality, which may still be desirable to have operating.
Accordingly, there is a need for an improved method for bypassing a supplemental active steering functionality, without bypassing a primary active steering functionality, in appropriate circumstances. There is also a need for an improved active steering system that is configured to bypass a supplemental active steering functionality, without bypassing a primary active steering functionality, in appropriate circumstances. There is a further need for an improved program product for an active steering system that is configured to bypass a supplemental active steering functionality, without bypassing a primary active steering functionality, in appropriate circumstances. Furthermore, other desirable features and characteristics of the present invention will be apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.